1. Field of the Invention
This invention relates to a device for heat fixation, and more particularly, to a device for heat fixation having an improved heating member suitable for electrophotographic apparatuses.
2. Description of the Prior Art
There are known various types of electrophotographic copying processes such as a process comprising forming an electrostatic latent image on a photoconductive material such as zinc oxide and the like coated on ordinary paper and developing the latent image directly and a process comprising forming an electrostatic latent image on a photoconductive material such as selenium, cadmium sulfide and the like mounted on a plate or drum, developing the latent image and transferring the developed image to a transfer member such as paper.
These electrophotographic processes suffer from the problem of heat fixation of the toner.
For example, visible images formed on a photosensitive plate are transferred to a copying member such as paper to produce transferred images and then heated for fixing the transferred images. When the toner of the image is composed of thermoplastic resin powders containing a coloring pigment, the toner is plasticized and softened by heating and then solidified. When the toner is a liquid toner containing a coloring pigment and thermoplastic resin in a solvent, the solvent is vaporized and dried by heat and as a result, the thermoplastic resin powders become sticky and are adhered to each other followed by solidifying as the copying member cools, to finish the fixation of the transferred images. Usually these copying steps are automatically carried out, that is, the copying member is transferred in the copying machine repeating a zigzag and a curved surface motions. Naturally, such complicated curved surface motions are continued in the fixing step and the heat fixation should be made under such motions. In general, the heat fixation is effectively conducted by closely contacting the copying member bearing the toner images with a heating surface of the fixing apparatus. Therefore, sometimes the heating surface has a complicated surface in accordance with the copying member moving in a zigzag way or a complicated curved surface way. Furthermore, it is necessary that the temperature distribution on the heating surface is uniform so as to prevent irregular fixation of the images and in addition, sometimes it is also necessary that a particular temperature distribution be maintained depending upon the required reproduction characteristics. Further important characteristic required in a heat fixing device is heating efficiency, that is,
(1) The period of time from switch-on of the power source to reaching a desired temperature of the heat fixing surface is short;
(2) For saving power and minimizing the size of the copying machine, heat efficiency should be high.
(3) High heat efficiency results in reducing the size of the fixing device, and the small fixing device is usually of small heat content and this results in a small heat transfer to parts around the fixing device and thereby temperature of those parts is not raised so that any heat resistant or heat shielding treatment is not necessary.
In view of the foregoing, heretofore there have been used metal, quartz or heat resistant plastics for the heating surface and a copying member is closely contacted with the heating surface for the purpose of enhancing the fixation efficiency. In such case, unnecessary developer (or toner) remains on the heating surface and dirties the subsequent copying members to disturb obtaining clear images.
An example of heating members used in conventional heat fixing devices is as shown in FIG. 1(a) and (b). In FIG. 1, there is provided a heating member 2 forming a heating circuit such as shown in FIG. 1(a) on a heat resistant insulating base 1 and the heating member 2 is coated with a heat resistant insulating layer 3 and further with a heat conductive layer 4 such as metal plates. The surface of heat conductive layer 4 of the heating member is contacted with a developed photosensitive paper or a copying member P such as a transfer member to which developed images have been transferred and thereby the developer is subjected to heat fixation.
In the conventional heating member, the temperature distribution on the surface of the heating member should be uniform throughout the whole surface and therefore a highly heat conductive one is required and usually a metal is used. However, a metal is, in general, electroconductive so that it should be insulated from the heat generating member and a heat resistant and electrically insulating layer 3 is provided. In general, electrocally insulating material are thermally poor conductor so that it takes a long time until heat reaches a copying member P and the heat efficiency is very low. In FIG. 1(b), 5 and 5' represent rollers conveying the copying member P in the direction of the arrow and BL represents air pressure for urging the copying member closely to the heating member.
After a developing step, the copying member P is usually transferred to a fixing step with a developing agent on a surface A and a part of the developing agent is often transferred from copying member P to rollers 5 and 5', and then transferred onto heating surface 4 and remains and adheres there. When copying is carried out continuously, the adhered unnecessary developing agent adheres to a surface B of the subsequent copying member to form unnecessary images there and therefore the copying member is dirtied. In particular, when two-side copying (on surfaces A and B) is carried out, the images are directly dirtied and this is very disadvantageous.
Another conventional heat fixing apparatus is of a roller type as shown in FIG. 2. A copying member P passes between two heating rollers 6 and 6' rotating in the direction of the arrow and the developer is fixed. Each of rollers 6 and 6' is composed of a shaft 7, a heat generating member 8, and a heat conductive cylinder 9. Between heat generating member 8 and heat conductive cylinder 9 there is provided an air layer 10 of high specific heat so that heat conduction is poor and the heat supply from heat generating member 8 to heat conductive cylinder 9 is effected only by radiation. Thus, the heat efficiency is very low.